SERIES A AND SERIES B PREFERRED UNIT PURCHASE AGREEMENT

Exhibit 2.1.4

SERIES A AND SERIES B PREFERRED UNIT PURCHASE AGREEMENT

by and among

Maxygen, Inc., a Delaware Corporation

Astellas Bio Inc., a Delaware Corporation

and Perseid Therapeutics LLC, a Delaware Limited Liability Company

Dated as of September 18, 2009

TABLE OF CONTENTS

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TABLE OF CONTENTS

(continued)

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SERIES A AND SERIES B PREFERRED UNIT PURCHASE AGREEMENT

THIS SERIES A AND SERIES B PREFERRED UNIT PURCHASE AGREEMENT is made as of the 18 day of September, 2009 by and among Perseid Therapeutics LLC, a Delaware limited liability company (the “ Company ”), on the one hand, and Maxygen, Inc., a Delaware corporation (“ Maxygen ”), and Astellas Bio Inc., a Delaware corporation (“ Bio ,” and together with Maxygen, the “ Purchasers ”), on the other hand. All capitalized terms used and not defined herein shall have such meanings as set forth in the Master Joint Venture Agreement among Maxygen, Bio, and Astellas Pharma Inc., a Japanese corporation, dated as of June 30, 2009 (the “ Joint Venture Agreement ”).

RECITALS

WHEREAS, the Purchasers are parties to the Joint Venture Agreement, and it is a condition to the closing of the transactions contemplated by such Joint Venture Agreement that the Company and the Purchasers execute and deliver this Agreement.

WHEREAS, the purchase and sale of the Units pursuant hereto shall be effectuated, first, by the contribution of certain Maxygen assets to the Company in exchange for equity in the Company and, immediately following the contribution of such assets to the Company, on Bio’s and Maxygen’s contribution of cash to the Company in exchange for equity in the Company.

NOW THEREFORE, the parties hereby agree as follows:

ARTICLE 1

PURCHASE AND SALE OF PREFERRED UNITS

1.1 Sale and Issuance of Series A and Series B Preferred Units .

(a) Subject to the terms and conditions of this Agreement, Maxygen agrees to purchase at the Closing (as defined below), and the Company agrees to sell and issue to Maxygen at the Closing, TEN MILLION (10,000,000) Series A Preferred Units (the “ Series A Preferred Units ”), at a purchase price of $1.00 per unit for an aggregate purchase price of TEN MILLION U.S. Dollars ($10,000,000), and FORTY MILLION (40,000,000) Series A Preferred Units in exchange for the Contributed Assets in a transaction intended to qualify as a tax-free exchange under Section 351 of the Code. The Series A Preferred Units issued to Maxygen pursuant to this Agreement shall be referred to as the “ Series A Units .”

(b) Subject to the terms and conditions of this Agreement, Bio agrees to purchase at the Closing, and the Company agrees to sell and issue to Bio at the Closing, TEN MILLION (10,000,000) Series B Preferred Units (the “ Series B Preferred Units ”), at a purchase price of $1.00 per unit for an aggregate purchase price of TEN MILLION U.S. Dollars ($10,000,000). The Series B Preferred Units issued to Bio pursuant to this Agreement shall be referred to as the “ Series B Units ,” and, along with the Series A Units, as the “ Units .”

1.2 Closing; Delivery .

(a) The closing of the transactions contemplated by this Agreement (the “ Unit Purchase ”) shall take place at the Closing as specified in Section 3.1 of the Joint Venture Agreement.

(b) At the Closing, the Company shall deliver to each Purchaser a certificate representing the Units being purchased by such Purchaser at such Closing against payment of the purchase price therefor by check payable to the Company or by wire transfer to a bank account designated by the Company (for the Units to be purchased by cash) or by transfer of the Contributed Assets pursuant to the Asset Contribution Agreement (for the Units to be purchased by contribution of the Contributed Assets), all as set forth in Section 1.1 .

1.3 Use of Proceeds . The Company will use the proceeds from the sale of the Units for general corporate purposes.

ARTICLE 2

REPRESENTATIONS, WARRANTIES AND COVENANTS OF THE COMPANY

The Company hereby represents, warrants and covenants to each Purchaser that, except as set forth on the Disclosure Schedule attached as Exhibit A to this Agreement, which exceptions shall be deemed to be part of the representations, warranties and covenants made hereunder, the following representations are true and complete as of the date of the Closing, except as otherwise indicated.

2.1 Organization and Operation . The Company is a limited liability company duly formed, validly existing and in good standing under the laws of the State of Delaware, the jurisdiction of its formation. The Company has not conducted any business operations prior to the date hereof.

2.2 Capitalization . The authorized capital of the Company (the “ Company Units ”) consists, immediately prior to the Closing, of:

(a) Seventy-five million (75,000,000) common units (the “ Common Units ”), none of which are issued and outstanding. All of the outstanding Common Units have been duly authorized, are fully paid and non-assessable and have been issued in compliance with all applicable federal and state securities laws.

(b) Sixty million (60,000,000) preferred units (the “ Preferred Units ”), of which fifty million (50,000,000) units have been designated Series A Preferred Units and ten million (10,000,000) units have been designated Series B Preferred Units, none of which are issued and outstanding immediately prior to Closing. The rights, privileges and preferences of the Preferred Units are as stated in the LLC Agreement and as provided by the Limited Liability Company Act of the State of Delaware (the “ LLCA ”). Immediately following the Closing, the only issued and outstanding Company Units will be all of the Preferred Units.

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(c) The Company has reserved fifteen million (15,000,000) Common Units for issuance to officers, managers, employees and consultants of the Company pursuant to the Equity Incentive Plan duly adopted by the Board of Managers and approved by the Company’s members. All such reserved Common Units remain available for issuance to officers, managers, employees and consultants pursuant to the Equity Incentive Plan. The Company has furnished to the Purchasers complete and accurate copies of the Equity Incentive Plan and forms of agreements to be used thereunder.

(d) Except for (i) the conversion privileges of the Units to be issued under this Agreement and (ii) the rights provided in the LLC Agreement and the Transaction Agreements, there are no outstanding options, warrants, rights (including conversion or preemptive rights and rights of first refusal or similar rights) or agreements, orally or in writing, to purchase or acquire from the Company any Common Units, Series A Preferred Units or Series B Preferred Units, or any securities convertible into or exchangeable for Common Units, Series A Preferred Units or Series B Preferred Units.

(e) Except as set forth in Section 2.2 of the Disclosure Schedule, none of the Company’s unit purchase agreements or unit award documents contains or will contain a provision for acceleration of vesting (or lapse of a repurchase right) or other changes in the vesting provisions or other terms of such agreement or understanding upon the occurrence of any event or combination of events. Except as set forth in the LLC Agreement or the Equity Incentive Plan, the Company has no obligation (contingent or otherwise) to purchase or redeem any of its membership units.

2.3 Authority . The Company has all necessary power and authority to execute and deliver this Agreement, to perform its obligations hereunder and thereunder and to consummate the Unit Purchase. The execution and delivery of this Agreement and the consummation by the Company of the Unit Purchase have been duly and validly authorized by all requisite action, and no other proceedings on the part of the Company are necessary to authorize this Agreement or to consummate the Unit Purchase. At the Closing, this Agreement will (a) be duly and validly executed and delivered by the Company and (b) constitute the legal, valid and binding obligations of the Company, enforceable against the Company in accordance with its terms, except as may be limited by bankruptcy, insolvency, reorganization, moratorium and other similar laws and equitable principles related to or limiting creditors’ rights generally, by the availability of equitable remedies and defenses, and to the extent the indemnification provisions contained in the Investors’ Rights Agreement may further be limited by applicable laws and principles of public policy.

2.4 Valid Issuance of Units . The Units, when issued, sold